Arc furnace control system



y 8, 1957 o. s. STEELE 2,794,142

ARC FURNACE CONTROL SYSTEM Filed Jan. 6, 1955 lnveniorl- Owen S. Steel by,

His AH'orney British Thomsonliouston Company, Limited, a Bi .ish company Application .lanuary 6, 1355, Serial No. 488,166

fliaims. (Cl. 314-41) This invention relates to arc furnaces and has for its principal object the provision of improved control means for automatically controlling the position of movable electrodes in such furnaces.

It is another object of this invention to provide an improved control means for automatically positioning movable electrodes in arc furnaces in accordance with a preselected relationship between arc voltage and are current.

It is still another object of this invention to provide an improved control arrangement for are melting furnaces, which improved control accomplishes the automatic positioning of movable electrodes in such furnaces in a continuous or modulating manner without the employment of circuit interrupting switches or contacts.

Briefly stated, this invention contemplates, in accordance with one aspect thereof, the provision of means for deriving and comparing signals proportional respectively to the arc voltage and the arc current. Associated with these last mentioned means is a saturable reactor control arrangement and a reversible electric motor controlled thereby for driving and positioning the movable electrode. The saturable reactor control is actuated responsive to deviations of arc voltage or are current or both of them, from a preselected relationship and operates to control the speed and direction of rotation of the electrode positioning and driving motor to continuously maintain the aforementioned desired arc condition.

In the case of the consumable electrode type furnace employed for melting purposes where one electrode is formed of the metal to be melted and the other electrode comprises the molten metal, the control serves to feed the consumable electrode into the furnace at a rate selected by the control in response to' a preselected arc current, are voltage relationship. When applied, on the other hand to an arc melting furnace of the non-consumable electrode type where the metal to be melted is independ ently introduced into the furnace and, in its melted state, forms one electrode and interacts with a non-consumable electrode, the control acts'to position the non-consumable electrode relative to the molten metal which forms the other electrode. This last mentioned control action may involve, for instance, a continuous retraction of the nonconsumable electrode to allow for the build-up of molten metal in a crucible, or, in the event that it is desired to continuously withdraw the melted metal from the crucible as it solidifies, the control action may result in adjustment of the movable electrode both toward and away from the molten pool to maintain the preselected relationship be tween arc voltage and are current.

It will be seen therefore that this invention has application in various forms thereof to different types and varieties of arc furnaces wherein automatic position control of movable electrodes 'is'desired.

' Other objects and advantages of this invention will be apparent from the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

United States Patent In the drawing, Fig. '1 is a schematic illustration of an arc furnace electrode control arrangement embodying this invention and employing a three phase motor elec* trode driving and positioning means; while Fig. 2 is a schematic illustration of another embodiment of this invention utilizing a single phase motor electrode drive.

Referring now to Fig. l, a reversible three phase motor 1 is provided to drive and position a movable electrode 30 in an arc furnace. The motor 1 may be connected to drive the electrode 30 in any suitable manner, such as through gearing and the like as indicated schematically at 31, so as to be capable of adjusting and driving the electrode in either direction. Numerous arrangements for mechanically coupling furnace electrodes and their drive motors are well known in the art. The motor 1 is connected so as to be energizable in a manner hereinafter described from a three phase alternating current source indicated at supply terminals L1, L2 and L3.

Four saturable reactors 2, 3, 4, and 5 are provided to effect control of the speed and direction of rotation of the motor 1. The reactors are provided with alternating current windings 2a, 3a, 4a and 5a respectively, which are connected in the lines leading from terminals L1 and L2 as shown in Fig. 1, and are further provided with control windings 2b, 3b, 4b and 512 respectively, which are on ergized in response to a relationship between arc voltage and are current as later set forth.

The A.-C. windings 2a of reactor 2 are connected in the conductor leading from supply terminal L1 to motor energizing terminal 6 While the A.-C. windings 3a of reactor 3 are connected in the line running from the sup' ply terminal L2 to motor energizing terminal 7. The A.-C. windings 4a and 5a of reactors 4 and 5 are connected to complete electrical circuits between the supply terminals L1 and L2 and the motor terminals 7 and 6 respectively. Motor energizing terminal 8 is connected directly to supply terminal L3. The control windings 2b, 3b, 4b and 5b of reactors 2, 3, 4 and 5 are differentially wound, as shown, in order to minimize the net effect of voltages induced in the control windings by the A.-C. windings.

The control windings 2b and 3b of reactors 2 and 3 are connected in series with each other and with a unidirectional impedance 9 across a pair of terminals 10 and 11. The control windings 4b and 5b of reactors 4 and 5 are connected in series with a unidirectional impedance 12 and also across terminals 10 and 11 but in a manner so as to be energizable by a voltage of opposite polarity at the terminals 10 and 11 from that capable of energizing windings 2b and 3b,

It can be seen from the drawing that when none of the control windings of the reactors 2, 3, 4 and 5 are energized, the impedances of the corresponding A.-C. windings 2a, 3a, 4a and So will be relatively very high and that consequently, the voltage appearing at the motor terminals 6, 7 and S will be a minimum. The reactors are selected such that, under the condition just referred to, the voltage appearing at the motor terminals 6, 7 and 8 is insutficient to cause rotation of the motor.

Upon energization of the control windings 2b and 3b of reactors 2 and 3, however, the impedances of the corresponding A.-C. windings 2a and 3a will decrease as an inverse function of the control current. The reduction in impedance decreases the voltage drop across the windings 2a and 3a and hence increases the voltage at the motor terminals 6, 7 and 8. The design of reactors 2 and 3 is such that some time prior to complete saturation, the voltage appearing at motor terminals 6, 7 and 8 is sufiicient to cause rotation of the motor and under such conditions the motor will begin to rotate in one direction pletely balanced out.

at a speed determined by the voltage drop in the reactor windings 2a and 3a.

On the other hand, when the control windings 4b and 5b are energized, it will be seen that the effect of such energization is to connect the supply terminals L1 and L2 to the motor terminals 7 and 6 respectively, which is the reverse of the connections made by energizing windings 2b and 3b. Therefore, upon energization of windings 4b and 5b the motor is caused to rotate in the opposite direction from that achieved by energization of windings 2b and 3b.

7 As will now be described the control windings 2b and 3b or 4b and 5b are energized when the relationship between a'rc voltage and electrode arc current deviates from some preselected value. Referring to the circuit of Fig. 1, a voltage V; is derived from a transformer connected in theelectric supply circuit for the electrode and is substantially proportional to the magnitude of current flowing in the electrode. This voltage V is fed to the primary of a transformer 13 which has a tapped resistance 14 connected across the primary to enable the preselection of the magnitude of the voltage signal proportional to the electrode current. Another voltage V is derived from the voltage across the arc, a tapped resistance 15 being included in series therewith to allow preselection of the magnitude of the voltage signal proportional to the arc drop.

These alternating voltages V0 and Va. are rectified in a pair of bridge type rectifiers 16 and 17 resp ctively. The voltage derived from rectification in the rectifier 16 is applied across a resistor 18 and the voltage proportional to the arc voltage drop as derived from the rectifier 17 is applied across a resistor 19 and a series connected slide wire resistor 20. V

The terminal 10 is connected as shown to a point between resistors 19 and 20 and to a slidable contact 21 while the terminal 11 is connected to the outer end of the resistor 18 through a drum controller 22 which, in the position shown, connects the circuit for automatic operation. Once the desired relationship or ratio between arc voltage and arc current has been selected and the voltages proportional to these quantities impressed on the resistors 18, 19 and 20 as shown, the slidable contact 21 is moved along the resistor 20 until the voltage appearing across the contacts 10 and 11 has been com- Thus, as longas the preselected relationship continues, no voltage signal will appear across the terminals 10 and 11.

A deviation in either direction however from the preselected relationship or ratio will cause a voltage to appear across terminals 10 and 11, the polarity of which depends on thedirection of the deviation. For instance,

assume that the arc voltage drop, as represented by Va, increases without a corresponding increase in th voltage signal proportional to are current; The voltage drop across the resistor 19 becomes gr'eaterthan the voltage drop across the resistor 18. The result is that the terminal 10 is at a lower potential than the terminal 11 so that in effect the terminal 11 is positive with respect to terminal 10.

With the terminal 11 positive, current will be caused to flow through the control windings 4b and 5b and through the unidirectional impedance 12 back to the terminal 11, thus decreasing the impedance of A.-C. windings 4a and 5a to cause the motor 1 to rotate in a direction to correct the error.

Assume, on the other hand, that the deviation is in the opposite direction, that is, that the magnitude of the arc current V(: increases above the preselected relationship with respect to arc voltage. Under such a condition the current flow through resistor 18 will be increased thus increasing the voltage drop across 1 8 and reducing the potential of terminal 11 below that of terminal 10. Terminal 10 is thus positive with respect to terminal 11 and the current therefore will flow through the unidirectional impedance 9 through the control windings 2b and 3b back to terminal 11. As a result, the impedance of A.-C. windings 2a and 3a is reduced as a function of the magnitude of the control current in the windings 2b and 3b so that the motor is caused to rotate in the opposite direction from that in the case just previously described.

It is apparent that when the potential appearing between terminals 10 and 11 has one polarity a pattern of energizing voltages is provided to the motor 1 to cause it to rotate in one direction and when the polarity of the potential is reversed another pattern of energizing voltages is provided to the motor to cause it to rotate in the opposite direction.

A transformer 23 and a rectifier unit 24 may be provided along with the drum control 22 to permit manual adjustment of the movable electrode position. As just described, with the controller in the position shown, the current is connected for automatic operation. As shown in the diagram, movement of the drum one step to either the right or the left from the position shown disconnects the terminal 11 from the resistor 18 so that the control circuit is in the off position.

Movement of the drum two steps to the right from the position shown completes a circuit to cause raising of the electrode. In the embodiment shown, the movable electrode is above its cooperating electrode and it will be seen that the polarity chosen for raising the electrode is with the terminal 10 positive with respect to terminal 11, which is the same polarity resulting from a relative increase in arc current or a relative decrease in arc voltage drop, or from both of these effects. On the other hand movement of the controller drum two steps to the left from the position shown connects the circuit such that terminal 11 is positive with respect to terminal 10, and in the embodiment illustrated, such a polarity corresponds to that requiring a lowering of the movable electrode to decrease the length of the arc gap,

Referring now to the embodiment of this invention shown in Fig. 2, a reversible single phase capacitor type motor 24' is employed instead of a three phase motor as in the arrangement illustrated in Fig. l. The motor is provided with a phase shifting capacitor 24a and is energized from a single phase source shown at terminals L1 and L2, the circuit being controlled by a pair of saturable reactors 25 and 26, the control windings 25a and 26a thereof being connected so as to be energizable from terminals 10 and 11 corresponding to terminals 10 and 11 in Fig. l. a

The reactor A.-C. winding 25b is connected in the line running from supply terminal L1 to motor terminal 28 and A.-C. winding 26b is connected between supply terminal L1 to motor terminal 27. Thus energization of control winding 25 causes the motor to rotate in one direction while energization of control winding 26a causes rotation of the motor in the opposite direction.

It will be realized that in certain applications where, for instance, it may be desired to continuously feed an electrode in one direction and the control is to perform the function of regulating the motor speed, it will be unnecessary to provide a reversible motor and the control may be simplified to avoid the provision of the additional reactors otherwise required to perform this function.

Further, it will be understood that this invention is applicable to various types of arc furnaces, including those employing direct current energized arcs wherein direct current signals proportional to are voltage and are current may be connected through suitable adjusting resistors either directly to first the reactor control windings orthrough suitable amplification means and then to the reactor control windings.

It will also be observed that this invention, as seen in one embodiment thereof, reqnires no external reference signal in the accomplishment of the electrode positioning control and that the automatic control fnnction is continuous and uninterrupted in operation, being ac- A rams complished entirely without the employment of electrical switches or contacts.

What I claim as new and desired to secure by Letters Patent of the United States is:

1. A control system for automatically adjusting the position of a movable arcing electrode in an arc furnace to vary the length of an are, said control system comprising an alternating current motor mechanically connected so as to be capable of driving and positioning said electrode, an electric power source for energizing said motor, saturable reactor means electrically connected between said power source and said motor to control the speed and direction of rotation of said motor, means for obtaining a signal substantially proportional to the magnitude of the current flowing in said arc, means for obtaining a signal substantially proportional to the mag nitude of the voltage drop across said arc, and means for comparing said are current and are voltage proportional signals including means for energizing said reactor means responsive to variations in said signals from a preselected relationship, whereby said motor is controlled and said electrode adjusted to restore said relationship.

2. A control system for automatically adjusting the position of a movable electrode in an arc furnace to thereby control the length of an arc, said control system comprising a reversible alternating current motor mechanically connected so as to be capable of driving and positioning said electrode in either direction, an alternating current electric power source for energizing said motor, saturable reactor means electrically connected between said power source and said motor, said reactor means being energizable to control the magnitude and the phase rotation of the alternating voltage applied to said motor, means for deriving a voltage substantially proportional to the magnitude of the current flowing in said arc, means for deriving a second voltage substantially proportional to the magnitude of the voltage drop across said arc, means for comparing the magnitudes of said are voltage and are current proportional voltages, and means for energizing said reactor means responsive to variations in the relative magnitudes of said are voltage and arc current proportional voltages from a preselected relationship, whereby the direction of rotation and speed of said motor is controlled and said electrode adjusted to restore said relationship.

3. A control system for an arc furnace having a movable electrode therein and a reversible alternating current electric motor for adjacent the position of said electrode to vary the length of an arc, the system comprising means responsive to voltage across said are and current through said are for providing unidirectional potentials whose polarities and amplitudes vary in accordance with departure of a ratio of said voltage and current from a predetermined value, and a plurality of saturable reactors having direct current control windings connected to receive said unidirectional potentials and alternating current windings for connection between said electric motor and an alternating current power source, said reactors being arranged to cause said motor to rotate in one direction in response to said unidirectional potentials having one polarity and in the opposite direction in response to said potentials of opposite polarity.

4. A control system for an arc furnace having a movable electrode therein and a reversible alternating current electric motor for adjusting the position of said electrode to vary the length of an arc, the system comprising means responsive to voltage across said are and current through said are for providing unidirectional potentials whose polarities and amplitudes vary in accordance with departure of a ratio of said voltage and current from a predetermined value, and a plurality of saturable reactors having direct current control windings connected to receive said unidirectional potentials and alternating current windings for connection between said electric motor and an alternating current source, said reactors being arranged to provide one pattern of energizing voltages to said motor in response to said unidirectional potentials of one polarity and another pattern of energizing voltages in response to said unidirectional potentials of the other polarity, whereby said motor rotates in one direction in response to said unidirectional potentials of one polarity and in the reverse direction in response to said unidirectional potentials of the opposite polarity.

5. A control system for an arc furnace having a movable electrode therein and a reversible alternating current electric motor for adjusting the position of said electrode to vary the length of an are, said motor having three energizing terminals, the system comprising means responsive to voltage across said arc and current through said are for providing unidirectional potentials whose polarities and amplitudes vary in accordance with departure of a ratio of said voltage and current from a predetermined value, and a plurality of saturable reactors having direct current control windings connected to receive said unidirectional potentials and al ternating current windings, the alternating current winding of at least one said reactor being connected between each of a pair of said energizing termnials and an alternating current power source, said reactors being arranged to cause said motor to rotate in one direction in response to said unidirectional potentials of one polarity and in the opposite direction in response to said unidirectional potentials of opposite polarity.

6. A control system for an arc furnace having a movable electrode therein and three-phase reversible electric motor for adjusting the position of said electrode to vary the length of an are, said motor having three energizing terminals for connection to three alternating current supply terminals, the system comprising means responsive to voltage across said are and current through said are for providing unidirectional potentials whose polarities and amplitudes vary in accordance with departure of a ratio of said voltage and current from a predetermined value, and a plurality of saturable reactors having direct current control windings connected to receive said unidirectional potentials and alternating current windings for connection between a pair of said supply terminals and a pair of said motor energizing terminals to cause current to how in one direction between said pair of energizing terminals in response to said unidirectional potentials of one polarity and in the opposite direction in response to said unidirectional potentials of opposite polarity.

7. A control system for an arc furnace having a movable electrode therein and a three-phase reversible electric motor for adjusting the position of said electrode to vary the length of an arc, said motor having three energizing terminals for connection to three alternating current supply terminals, the system comprising means responsive to voltage across said are and current through said are for providing unidirectional potentials whose polarities and amplitudes vary in accordance with departure of a ratio of said voltage and current from a predetermined value, and a plurality of saturable reactors each having a direct current control winding and an alternating current winding, the alternating current windings of a first pair of said reactors being connected between one said supply terminal and a pair of said energizing terminals, the alternating current windings of a second pair of said reactors being connected between another said supply terminal and said pair of energizing terminals, the control Winding of one reactor of each said pair being connected to be energized by said unidirectional potentials of one polarity and the control winding of the other reactor of each said pair being connected to be energized by said unidirectional potentials of the other polarity.

(References on following page) Referenc eg Cited in the file of this patent UNITED STATES PATENTS 8 Wickerham Apr. 27, 1948 lourn eau x June30, 1951 Wickerham et a1 Aug. 21, 1951 McKenney et a1 Feb. 7, 1956 Crenshaw June 12, 1956 

